The present invention relates to a device and a method for spatially resolved measurement of temperature and/or strain by Brillouin scattering.
Brillouin scattering in optical fibers can be used for a distributed or spatially resolved measurement of temperature and strain along the optical fiber, because the frequency and the amplitude of the Brillouin scattering are a function of the measurement parameters temperature and strain (see: Galindez-Jamioy & López-Higuera, 2012 Brillouin Distributed Fiber Sensors: An Overview and Applications. 2012, 17).
Frequently, only the Brillouin frequency is measured, which depends very profoundly on the measurement parameters, for example, with about 1 MHz/° C. or 0.05 MHz/μϵ in quartz glass and which can be determined very accurately. However, separating the influence of both measurement parameters is problematic.
The two measurement parameters can be separated in some situations by comparative measurements on differently installed optical fibers, for example loose tubes with a loose fiber or a tight tube with a fixed fiber (see: Inaudi & Glisic, 2006 Reliability and field testing of distributed strain and temperature sensors 6167, 61671D-61671D-8). Alternatively, measurements of the Brillouin frequencies either in fibers with multiple Brillouin peaks (see: Liu & Bao, 2012 Brillouin Spectrum in LEAF and Simultaneous Temperature and Strain Measurement Lightwave Technol., 30 (8), 1053-1059) or in oligo-mode fibers with few different spatial modes (see: Weng, Ip, Pan, & Wang, 2015, Single-end simultaneous temperature and strain sensing techniques based on Brillouin optical time domain reflectometry in few-mode fibers, Opt. Express, 23 (7), 9024-9039) with different dependencies of the frequency on temperature and strain can be used to separate the measurement parameters.
However, all these methods cannot be widely used, because suitable optical fibers are not always available for the application. Furthermore, the installation and measurement of several optical fibers or of special fibers is associated with higher expenses.
Another method for separating the two measurement parameters is the measurement of frequency and amplitude of one or more Brillouin peaks (see: Parker, Farhadiroushan, Handerek, & Rogers, 1997, Temperature and strain dependence of the power level and frequency of spontaneous Brillouin scattering in optical fibers, Opt Lett., 22 (11), 787-789, and Maughan,. Kee & Newson, 2001, Simultaneous distributed fiber temperature and strain sensor using microwave coherent detection of spontaneous Brillouin backscatter, Measurement. Science and Technology, 12 (7), 834). In this way, two independent measurement parameters are obtained, from which both these physical parameters are can determined in principle. However, the dependence of the amplitude on the temperature and strain is weak and amounts, for example, to approximately 0.3%/° C. Therefore, the amplitude must be measured very precisely to achieve practically relevant temperature resolutions and accuracies of about 1° C.
A known method for improving the accuracy is to compare the Brillouin amplitude with the amplitude of Rayleigh scattering from the same fiber (see: Wait & Newson, 1996, Landau Placzek ratio applied to distributed fiber sensing, Optics Communications, 122, 141-146). The influence of fiber attenuation can be eliminated by calculating the ratio of the Brillouin amplitude to the Rayleigh amplitude, which is referred to as Landau Placzek ratio.
Usually, the Brillouin signal is not measured simply with an optical filter and a photodiode, because the required very narrow-band optical filters are difficult to produce and are thermally not very stable. Moreover, the alternative measurement of the Brillouin scattering can measure lower signal strengths with an optical heterodyne receiver (see: Maughan, Kee, & Newson, 2001). Brillouin scattering signal is hereby superimposed with laser light having the same frequency as the laser exciting the Brillouin scattering or a frequency shifted by several GHz (local oscillator LO). The photodetector then detects a superimposed signal with a frequency that corresponds to the difference between the Brillouin frequency and the laser frequency or LO frequency, respectively. When mixed with the exciting laser, the difference frequency for quartz glass is about 10 GHz, This signal is typically GHz mixed with an electronic local oscillator in order to obtain a better measurable difference frequency below 1 GHz (Shimizu, Horiguchi, Koyamada & Kurashima, 1994, Coherent self-heterodyne Brillouin OTDR for measurement of Brillouin frequency shift distribution in optical fibers, Lightwave Technology, Journal of, 12 (5), 730-736).
However, in addition to the fiber attenuation, there is the additional problem caused by the polarization dependence of the measured signal. This problem interferes with the accuracy of the measurement of both parameters, namely frequency and amplitude. When the Brillouin signal is superposed with an optical local oscillator, only the signal component that matches the polarization of the local oscillator is admixed to the difference frequency. The signal with another polarization is then lost for the measurement. In addition, the polarization of the Brillouin signal during transmission through the optical fiber changes due to the stress-induced birefringence in the optical fiber. This means that the amplitude of the measured polarization component of the Brillouin signal varies strongly as a function of the distance. This polarization dependence thus makes an accurate amplitude determination considerably more difficult and also degrades the accuracy of the frequency determination. Until now, attempts were made to compensate for this effect by averaging over measurements with different polarization of the exciting laser or local oscillator (see: Fan, Huang, & Li, 2009, Brillouin-based distributed temperature and strain sensor using the Landau-Placzek Ratio, 7381,738105-738105-9 and Song, Zhao, & Zhang, 2005, Optical coherent detection Brillouin distributed optical fiber sensor based on orthogonal polarization diversity reception, Chin. Opt. Lett., 3 (5), 271-274). However, a large number of averages are required for a reasonably accurate measurement, without having solved the problem of signal loss.
The problem forming the basis of the present invention is therefore to provide a device and a method of the aforementioned type, with which the temperature and the strain can be determined more easily and/or more precisely.